1. Field of the Invention
The present invention relates generally to an optical scanner and method of scanning with an extended depth of focus, and more particularly pertains to an optical scanner and method of scanning bar codes which achieves an extended depth of focus by employing a multifocal length optical system. The subject invention is particularly useful in an optical scanner employing a nonlaser radiation source such as a light emitting diode (LED) or a linear array of LEDs, but is also applicable to optical scanners employing a laser source such as a laser diode. The subject invention is very useful in CCD based bar code readers.
2. Description of the Related Art
Various optical readers and optical scanning systems have been developed heretofore for reading bar code symbols appearing on a label or on the surface of an article. The bar code symbol itself is a coded pattern of indicia comprised of a series of bars of various widths spaced apart from one another to bound spaces of various widths, the bars and spaces having different light reflecting characteristics. The readers and scanning systems electro-optically transform the graphic indicia into electrical signals, which are decoded into alphanumeric characters that are intended to be descriptive of the article or some characteristic thereof. Such characters are typically represented in digital form and utilized as an input to a data processing system for applications in point-of-sale processing. Inventory control, and the like scanning systems of this general type have been disclosed, for example, in U.S. Pat. Nos. 4,251,798; 4,369,361; 4,387,297, 4,409,470; 4,760,248; and 4,896,026, all of which have been assigned to the same assignee as the instant application.
As disclosed in some of the above patents, one embodiment of such a scanning system resides, inter alia, in a hand-held, portable laser scanning head supported by a user, which is configured to allow the user to aim the head, and more particularly, light beam, at a target and a symbol to be read.
The light source in a laser scanner is typically a gas laser or semiconductor laser. The use of a semiconductor devices such as a laser diode as the light source in scanning systems is especially desirable because of their small size, low cost and low power requirements. The laser beam is optically modified, typically by a lens, to form a beam spot of a certain size at the target distance. It is preferred that the beam spot size at the target distance be approximately the same as the minimum width between regions of different light reflectivity, i.e., the bars and spaces of the symbol.
Bar code symbols are formed from bars or elements that are typically rectangular in shape with a variety of possible widths. The specific arrangement of elements defines the character represented according to a set of rules and definitions specified by the code or "symbology" used. The relative size of the bars and spaces is determined by the type of coding used, as is the actual size of the bars and spaces. The number of characters per inch represented by the bar code symbol is referred to as the density of the symbol. To encode a desired sequence of characters, a collection of element arrangements are concatenated together to form the complete bar code symbol, with each character of the message being represented by its own corresponding group of elements. In some symbologies a unique "start" and "stop" character is used to indicate where the bar code begins and ends. A number of different bar code symbologies exist. These symbologies include UPC/EAN, Code 39, Code 128, Codabar, and Interleaved 2 of 5.
A further known symbology is known as two-dimensional (2D) symbology and is discussed in detail in commonly-assigned U.S. Pat. Nos. 5,243,655 and 5,304,786, which are incorporated herein by this reference thereto. Briefly, that symbology involves a variable number of component symbols or "codewords" per row of a nonvolatile electro-optical read-only memory imprinted on a substrate. Codewords in alternating rows are selected from mutually exclusive subsets of a mark pattern, the subsets being defined in terms of particular values of a discriminator function which is illustrated in the referenced patents as being a function of the widths of bars and spaces in a given codeword.
In the scanning systems known in the art, the light beam is directed by a lens or similar optical components along a light path toward a target that includes a bar code symbol on the surface. The scanning functions by repetitively scanning the light beam in a line or series of lines across the symbol. The scanning component may either sweep the beam spot across the symbol and trace a scan line across the past the symbol, or scan the field of view of the scanner, or do both.
Scanning systems also include a sensor or photodetector which functions to detect light reflected from the symbol. The photodetector is therefore positioned in the scanner or in an optical path in which it has a field of view which extends across and slightly past the symbol. A portion of the reflected light which is reflected off the symbol is detected and converted into an electrical signal, and electronic circuitry or software decodes the electrical signal into a digital representation of the data represented by the symbol that has been scanned. For example, the analog electrical signal from the photodetector may typically be converted into a pulse width modulated digital signal, with the widths corresponding to the physical widths of the bars and spaces. Such a signal is then decoded according to the specific symbology into a binary representation of the data encoded in the symbol, and to the alphanumeric character so represented.
The decoding process in known scanning systems usually works in the following way. The decoder receives the pulse width modulated digital signal from the scanner, and an algorithm implemented in software attempts to decode the scan. If the start and stop characters and the characters between them in the scan were decoded successfully and completely, the decoding process terminates and an indicator of a successful read (such as a green light and/or an audible beep) is provided to the user. Otherwise, the decoder receives the next scan, performs another decode attempt on that scan, and so on, until a completely decoded scan is achieved or no more scans are available.
Such a signal is then decoded according to the specific symbology into a binary representation of the data encoded in the symbol, and to the alphanumeric characters so represented.
Decoding in 2D symbology is discussed particularly and shown in various flowcharts set forth in the 2D symbology patents incorporated by reference and above identified.
Laser scanners are not the only type of optical instrument capable of reading bar code symbols. Another type of bar code reader is one which incorporates a detector based upon charge coupled device (CCD) technology. CCDs are an array of many detectors. The entire symbol is flooded with light from the reader or ambient light, and each CCD detector is sequentially read out to determine the presence of a bar or a space. Such readers are light-weight and easy to use, but require substantially direct contact or placement of the reader on the symbol to enable the symbol to be properly read. Such physical contact of the reader with the symbol is a preferred mode of operation for many applications, or as a matter of personal preference by the user.
The depth of focus of bar code scanners using Light Emitting Diodes (LEDs) is very limited as compared to laser based scanners. Depth of focus is limited typically by geometrical defocusing, which is linear (approximately with f#).
Efforts have heretofore been made to provide an optical scanner and method of scanning with an extended depth of focus, e.g., as in U.S. Pat. No. 5,210,398, which issued on May 11, 1993, and is assigned to the same assignee as the instant application. The '398 patent is incorporated herein by this reference thereto.
The approach of the '398 patent involves the use of a multifocal length collection optical system, having at least a near range focal length and a long range focal length. The optical system collects radiation from objects in the field of view thereof and directs it to an optical detector for detection and generation of an electrical output signal indicative thereof.
In the '398 patent, the optical detector is disposed along an optical axis of the optical system and receives radiation focused at both focal lengths of the optical system and signals corresponding thereto are summed. A high pass filter removes the d.c. level from summed signal, or the summed signal may be applied to a differentiator. The output of the high pass filter or the differentiator constitutes the output detector signal. In either instance, the output detector signal exhibits an improved depth of modulation in respects of different light reflectivity parts in the field of view.